1. Field of the Invention
This invention relates to methods and apparatus for protecting loudspeakers, and especially to methods and apparatus for controlling the drive signal supplied to the loudspeaker so as to avoid over-temperature operation.
2. Description of the Related Art
A number of different products include audio circuitry, such as an audio amplifier, together with one or more loudspeakers and/or connections for driving one or more loudspeakers of a peripheral apparatus such as a headset. In some instances the loudspeaker(s) chosen will be robust enough and large enough to handle the maximum power level at which the amplifier could drive signals continuously into it, even under the worst case environmental conditions, for instance maximum supply voltage, maximum ambient temperature etc. However having robust enough loudspeakers is not always economical, and for portable devices in particular the desire is typically to make the speaker as small and light as possible. This can potentially lead to the audio drive circuitry overloading the loudspeaker. One particular problem is thermal overload of the loudspeaker.
A typical loudspeaker comprises a diaphragm which is driven by a voice coil supported relative to a magnet. In use, typically, an analog audio drive signal is applied to the voice coil to drive the loudspeaker. FIG. 1a illustrates an electrical model of a loudspeaker voice coil. When a voltage Vspka is applied to the voice coil, a current Ispka flows. The voice coil impedance observed as defined by Vspka/Ispka comprises some inductance Lea, but at audio frequencies the ohmic resistance Rea of the coil winding dominates. It will be understood that power Pd is dissipated in the loudspeaker, primarily as ohmic losses (Pd=Ispka2*Rea=Vspka2/Rea) in the voice coil which can cause heating of the voice coil. One particular problem to be avoided is overheating of the voice coil which could result in degradation in performance and/or damage to the loudspeaker. In some applications therefore there may be speaker protection circuitry for controlling the loudspeaker operation to avoid the voice coil temperature exceeding a specified limit.
FIG. 1b illustrates a thermal model of a loudspeaker. The thermal mass of the voice coil is modelled by thermal capacitance Cthvc which is at temperature Tvc above a reference temperature value Tref, say 300K. As mentioned above ohmic power losses can result in heating of the voice coil and thus an increase in the voice coil temperature Tvc. Power (modelled as thermal current) flows through a thermal resistance Rthvcm to the adjacent magnet with thermal inertia represented by thermal capacitance Cthm and thence via further thermal resistance Rthma to the outside world, assumed to be an independently defined ambient temperature Ta. It will be appreciated that this is a relatively simple model and more complex models could be developed if required, for instance including other components of the loudspeaker and/or temperature gradients within components such as the magnet.
The audio driving circuitry may therefore be limited in terms of the output power it can deliver to the loudspeaker so that the maximum power dissipated in the voice coil when flowing through the various thermal resistances to the ambient temperature does not cause the voice coil temperature to exceed some specified safe limit. This approach however requires assumptions about the thermal resistances of the voice coil and magnet and the worst case maximum ambient temperature. This will typically lead to conservative values being assumed which may lead to limiting of the output power to undesirably low levels.
Uncertainties in the relevant thermal resistances of the loudspeaker could in some instances be reduced by a measurement/calibration step for instance by extracting a thermal resistance from temperature versus power measurements. However due to the long thermal time constants involved such measurements typically take many seconds to be performed and thus are not typically suitable for self-calibration, e.g. on start-up or reset of a host device. Such a test could be performed as an initial factory calibration but even then would greatly extend production test time and thus increase cost and may not be suitable for a high-volume manufacturing process.
Even if the thermal resistances are well characterised there may be uncertainty about the ambient temperature. The ambient temperature could be measured in use but a single measurement of ambient temperature may be insufficient to take into account thermal gradients in the host device whereas using multiple sensors may add to the complexity and expense of the audio circuitry.
An alternative approach is therefore to measure the voice coil temperature itself with some sort of power limiting applied if a temperature threshold is crossed. This approach has the advantage of working from an actual indication of voice coil temperature but it has been appreciated the limiting applied may result in needless attenuation of some signals in some circumstances as discussed below.